1. Field of the Invention
The invention relates to an epitaxially coated silicon wafer and a method for producing an epitaxially coated silicon wafer.
2. Background Art
The quality requirements for the edge of a silicon wafer are ever increasing, in particular for large diameter silicon wafers (diameter z 300 mm). In particular, the edge of the silicon wafer should be free of contamination as far as possible and have a low roughness. Moreover, the edge should be resistant to increased mechanical stresses during handling. The untreated edge of a silicon wafer sliced from a single crystal has a comparatively rough and nonuniform surface. It often experiences spalling under mechanical loading and is a source of disturbing particles. It is therefore customary to regrind the edge in order thereby to eliminate spalling and damage in the crystal and to provide it with a specific profile.
Suitable grinding equipment is known from the prior art. It is customary for the silicon wafer to be fixed on a rotating table and for its edge to be moved onto the likewise rotating working surface of a machining tool. The machining tools are generally in the form of disks which are secured to a spindle and have circumferential surfaces serving as working surfaces for machining the edge of the silicon wafer. The material removing abrasive grain is usually securely anchored in the working surfaces of the machining tools. These machining tools are suitable for providing the silicon wafer with a rounded edge. It is usual for a certain minimum roughness to remain on the edge surface after the edge rounding.
In a subsequent machining step, the edge of the silicon wafer, which has been ground and treated with etching medium, is usually polished. In this case, the edge of a centrally rotating silicon wafer is pressed against a centrally rotating polishing drum with a specific force (contact pressure). U.S. Pat. No. 5,989,105 discloses an edge polishing method of this type, in which the polishing drum consists of an aluminum alloy and is covered with a polishing cloth. The silicon wafer is usually fixed on a flat wafer holder, a so called “chuck”. The edge of the silicon wafer projects beyond the chuck, such that it is freely accessible to the polishing drum.
For process monitoring, after edge polishing, it is customary for a visual check of the edges of the silicon wafers to be carried out under a microscope on a sample basis. This check is for particles, roughness and defects on the edge of the silicon wafer. In particular, the edge of the silicon wafer is examined for light reflections caused by possible unevennesses. However, this visual check is unreliable and does not allow a uniform edge quality to be ensured for all silicon wafers. Furthermore, the visual check is unsuitable for completely and reliably detecting small defects in the edge region of a silicon wafer in order to draw conclusions as to a possibly defective procedure.
Inspection devices are available, but these generally only allow inspection of the silicon wafer as far as the wafer edge to an edge exclusion zone of 3 mm. Most known inspection apparatuses cannot be used to examine the entire edge region, that is to say both the edge of the silicon wafer and the edge exclusion zone. By way of example, EP 1348947 A1 discloses an inspection apparatus which can be used to inspect the edge of the silicon wafer, but not the entire edge region (edge exclusion). By contrast, DE 10352936 A1 describes an inspection apparatus which allows automatic detection of defects starting from a specific size in the entire edge region including the edge exclusion zone of the silicon wafer.
Previous edge inspection methods do not yield sufficient information to optimize fabrication processes accordingly. In particular, it is not possible to unambiguously differentiate between defects and thus to monitor and avoid the production of critical defects.